Summary Opioid drugs are the most widely used analgesics in clinic, and are also some of the most widely abused substances. The adverse actions of these drugs, including peripheral side effects, dependence and tolerance, severely limit their utility as prescription analgesics for long term pain management. The -opioid receptor (MOR) is the primary target of the analgesic and rewarding effects of opioids. Thus, efforts aimed at developing safer and more effective opioid treatments will require a much deeper understanding of MOR signaling. Our long-term goal is to use unbiased forward genetics to dissect the molecular organization of the MOR signaling network using whole-animal behavioral responses to opioids as a phenotypic readout. Towards this goal, we developed a transgenic MOR model (tgMOR), in which mammalian MOR is expressed in the nervous system of the nematode C. elegans. We found that tgMOR animals gain the ability to respond to opioids, and exhibit all the cardinal behavioral hallmarks of opioid responses seen in higher organisms including acute depressant effects, desensitization and tolerance. We further demonstrated key known molecular players that control opioid responsiveness in mammals play conserved functions in tgMOR worms. Taking advantage of this model, we completed an unbiased, forward genetic screen for modifiers of behavioral opioid sensitivity, and isolated a large number of mutants with altered opioid responses. We have developed a pipeline for discovery, identification and validation of genes responsible for phenotypes using a combination of whole genome sequencing, mapping and targeted CRISPR/Cas9 gene editing. Using this approach, we uncovered several known and novel genes that regulate opioid responsiveness in worms, and confirmed their effects on MOR signaling using cell-based assays with cultured mammalian cells. Our findings suggest an elaborate, largely unknown, network of players exists to regulate MOR signaling. Thus, the main effort of this project focuses on identifying and characterizing these players by analyzing tgMOR mutants isolated from our unbiased, forward genetic screen. Our first aim will be to identify the genes responsible for 1) hypersensitivity, 2) hyposensitivity, and 3) impaired tolerance by pursuing a subsets of mutants from each phenotypic category. In the second aim, we will validate and perform mechanistic studies on identified, conserved regulators of MOR signaling using a comprehensive platform of cell-based assays that monitor various aspects of MOR signaling. The third aim will focus on exploring the pharmacogenomics by which MOR impacts behavior. To do so, we analyze interactions between genetic MOR variants found naturally in the human population, FDA-approved opioid drugs, and different genetic backgrounds using a humanized tgMOR C. elegans platform. It is anticipated that these studies will advance our understanding of how opioids act thereby paving the way to the development of safer opioid therapeutics.